Recently, along with the trend for high density and high integration of a semiconductor device, its wiring structure becomes finer rapidly. In forming a fine wiring structure, it is important to form a hole or a trench having a high aspect ratio and a small opening dimension.
Various plasma etching methods have been proposed for forming a hole or a trench in, e.g., a silicon layer (Si layer). As for a plasma etching method, for example, as shown in FIG. 4A, while a silicon oxide film (SiO2 film) 1 having an opening 1A of a predetermined pattern is employed as a hard mask, the silicon layer (Si layer) 2 which is disposed underneath the SiO2 film 1 is etched in accordance with the opening 1A, whereby a hole or a trench 2A is formed as depicted in FIG. 4B. However, reaction products (e.g., SiBrxOy) generated in etching tend to be attached to the opening 1A of a mask, thereby forming deposits 3 as shown in FIG. 4B.
The deposits 3 formed of reaction products in the opening 1A of the mask tend to grow bigger as the hole or the trench becomes deeper to form more deposits 3 in the opening 1A, thereby making the opening 1A narrower as shown in FIG. 4B. As a result, an effective actual opening dimension d′ of the opening 1A becomes smaller and an apparent aspect ratio h/d′ increases. Thus, an etching rate in the hole or the trench 2A becomes low and a very long time is required for etching the hole or the trench of a desired depth. As the hole or the trench 2A gets deeper and the etching rate becomes slower, a sidewall of the hole or the trench 2A tapers away and, occasionally, an etching may be stopped altogether.
Therefore, the applicant of the present invention proposed a technology for preventing reaction products from being attached to the opening of the mask (see, e.g., Reference 1). This technology employs as an etching gas a processing gas containing a gaseous mixture in which one or both of SF6 gas and NF3 gas are added to HBr gas, O2 gas and SiF4 gas, wherein a gas containing C and F is further added to the processing gas. By controlling the timing of adding the gas containing C and F (e.g., CF4 gas) to the etching gas, the accumulation of deposits at the opening of the mask can be suppressed and a hole or a trench having a high aspect ratio can be achieved.
Further, Reference 2 discloses an etching method capable of enhancing an etching anisotropy. In this case, when the semiconductor substrate is etched through a mask which is formed on the semiconductor substrate and has a lid shaped section, etching particles obliquely incident upon the opening of the mask are introduced into the lid, whereby the etching particles can be prevented from being scattered on the sidewall of the trench, thereby realizing an anisotropic etching.
[Reference 1] Japanese Patent Laid-open Application No. 2004-304029
[Reference 2] Japanese Patent Laid-open Application No. H1-216536
However, in the technology disclosed in Reference 1, since the deposits are prevented from being formed in the opening of the hard mask by a gas containing C and F which is added to the processing gas, the gas containing C and F for preventing the deposits is necessary in addition to the main etching gas. Further, since a hard mask itself is reduced by the gas containing C and F, an added amount of the gas containing C and F and a timing of adding it should be controlled. That is, when forming the hole or the trench having a high aspect ratio and an opening dimension equal to or smaller than 0.2 μm, the deposits in the opening of the mask have to be removed or prevented from growing during the etching.
Further, Reference 2 discloses a technology for preventing etching particles from scattering by using a lid of a mask, but it does not mention an attachment of the reaction products or an undesirable effect caused thereby.